1. Field of the Invention
The invention relates generally to earth drills and more particularly to a drilling system wherein a self-propelled down-hole drilling tool is connected to the surface through a flexible umbilical.
2. Description of the Prior Art
Typically, drilling means use a drill string of pipe segments through which the torque necessary to operate a drill bit is transmitted. As the depth of the hole increases, power loss increases significantly mainly due to friction between the drill string and the bore wall. Another characteristic of conventional drills is the length of time it takes to remove the bit from the hole and return it. This is known as "trip time". When drill strings are used, trip time represents a significant nonproductive time.
Besides transmitting the torque necessary to rotate a drill bit, the weight of the drill string provides the axial force at the drill bit needed to force the drill bit into the formation to be drilled.
An early example of a device eliminating the drill string is shown in U.S. Pat. No. 1,118,001 issued to May where a pneumatically operated drill and anchoring system is disclosed. Another anchoring system using an expandable bladder to both seal the hole and provide an anchor is shown in U.S. Pat. No. 3,376,942 issued to VanWinkle.
In order for a drill system to be independent of a drill string, it must be able to propel itself within the drill hole. Most systems using this method use a combination of a thrusting device to put force on the drill bit and dual anchoring systems to permit the drill tool to advance within the hole as the hole is deepened by action of the drill bit. Examples of such systems are shown in U.S. Pat. No. 3,354,969 issued to Ebeling, U.S. Pat. No. 3,797,589 issued to Kellner, and U.S. Pat. No. 4,060,141 issued to Catterfeld. Other combinations of anchoring devices and thrusting devices are disclosed in U.S. Pat. No. Re. 28,449 issued to Edmond and U.S. Pat. No. 3,978,930 issued to Schroeder.